Apparatus for large-scale diamond polishing

ABSTRACT

An apparatus for the polishing of diamond surfaces, wherein the diamond surface is subjected to plasma-enhanced chemical etching using atomic oxygen polishing plasma source, is presented. In the apparatus, a magnetic filter passes a plume of high-density, low-energy, atomic oxygen plasma. The plasma is capable of uniformly polishing diamond surfaces utilizing low energy atomic oxygen ions to chemically etch a diamond surface at moderate temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This divisional application claims the benefit of priority toU.S. utility application No. 09/541,178, filed in the United States onApr. 3, 2000, entitled “Method and Apparatus for Large-Scale DiamondPolishing.”

FIELD OF THE INVENTION

[0002] This invention relates to an apparatus for diamond polishing.More specifically, it relates to the use of plasma-enhanced chemicaletching techniques for polishing a synthetic diamond to an opticalquality surface.

BACKGROUND

[0003] Extreme hardness, high thermal and chemical stability, andoptical transparency are properties that render diamonds desirable innumerous optical, electrical, and military applications. In order toovercome the rarity and cost of natural diamonds, synthetic methods ofdiamond preparation have been developed. Synthetic diamonds areefficiently and cost-effectively fabricated in the form of coatingsusing plasma-assisted (or plasma-enhanced) chemical vapor deposition(PACVD or PECVD) processes. As deposited, the diamond films arepolycrystalline, typically possessing a roughness on the order of 10 to20 micrometers. The rough surface negates the utility of syntheticdiamonds in many applications, particularly optics. When the as-preparedsynthetic diamonds are used as lens coatings, for example, their roughsurfaces produce excessive scatter and thus, provide low transmittance.Costly labor-intensive polishing must be performed in order to achievethe required finish for this type of application. Mechanical polishingtechniques utilizing diamond paste are typically performed, but becausethe synthetic diamonds have the same hardness as the diamond in thepaste, polishing must be performed repetitively and for an extendedperiod. As a result, the cost of polishing the synthetic diamond tooptical quality exceeds the cost of depositing the diamond layer.

[0004] To reduce the polishing time and cost, a repetitiveion-implantation mechanical polishing technique was designed and isdisclosed in U.S. Pat. No. 5,154,023. When utilizing this technique, therough diamond surface is first “softened” by the formation of anion-implanted layer. This softened layer is subsequently subjected tomechanical polishing. The softening and polishing steps are repeateduntil a desired surface smoothness has been achieved. Each cycle ofsoftening and polishing in this technique affects only a shallow surfacelayer (on the order of 0.1 microns), so dozens of cycles are necessaryto process a typical synthetic diamond to optical quality. Therepetitive ion implantation polishing technique requires high ionenergies (on the order of 100 keV) in order to achieve ion implantationin the diamond surface; this requirement contributes both to overallcost of the method and also raises potential safety issues. Because thesynthetic diamond surface has various grain orientations, line-of-sighteffects from the high-energy implantation can result in directionalsputtering on the surface, thus hampering the production of a smoothsurface. In addition, the ion implantation apparatus typically has asmall beam spot and therefore, repeated scanning of the beam over thesample is necessary to achieve uniform ion-implantation across thesurface of a large synthetic diamond.

[0005] Synthetic diamond films and wafers have been used in variousmicroelectronic applications, such as heat sinks or substrates forsemiconductor devices. In these applications, it is often desirable toimpart a specific architecture on the surface of the diamond. Oxygenplasma, coupled with pattern masking, has been utilized to selectivelyetch synthetic diamond wafers. Typically, the masked wafer is etched ina low-pressure oxygen gas reactor, using electromagnetic radiation togenerate an oxygen plasma. Under these conditions, etching of thediamond wafer is rapid compared to conventional mechanical polishingtechniques. However, under these conditions, the etching of the wafer isanisotropic, most likely due to physical bombardment by high-energymolecular oxygen ions, so polishing to optical quality smoothness is notfeasible.

SUMMARY

[0006] One object of the present invention is to overcome thedisadvantages of the processes described above by providing an apparatusfor rapid, uniform, and cost-effective synthetic diamond polishing. Morespecifically, one embodiment of the present invention provides anapparatus for effectively polishing a synthetically produced diamond byplasma-enhanced chemical etching using an atomic oxygen polishing plasmasource, said source generating high concentrations of low energy atomicoxygen ions over a large surface area. The present invention takesadvantage of the ability of low energy atomic oxygen ions to chemicallyetch a diamond surface at moderate temperatures. Because the atomicoxygen ions have low energy and high density, they conform to thesurface of the synthetic diamond sample, and thus polish the sample withincreased uniformity versus known oxygen etching techniques. The rate ofpolishing is proportional to the density of atomic oxygen, and, in thepresent invention, this density can be easily controlled by adjustingparameters such as gas pressure, discharge voltage, and plasma ionsource power to minimize the processing time.

[0007] Because the present invention utilizes a chemical effect topolish the diamond surface, the energy of the atomic oxygen ionsgenerated is much lower than the energy of ions generated during ionimplantation techniques or similar high energy beam approaches.Accordingly, the atomic oxygen polishing plasma source disclosed hereincan operate at lower voltages than the apparatus for ion-implantation,thereby reducing both capital investment and safety concerns. Inaddition, because the atomic oxygen polishing plasma source generates alarge plume of plasma, large diamond samples can be polished in theirentirety without beam scanning, and multiple samples can be polishedsimultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a schematic representation of the atomic oxygenpolishing plasma source of the present invention;

[0009]FIG. 2 is a schematic representation of the plasma composition inthe different regions of the atomic oxygen polishing plasma source andvacuum chamber of the present invention;

[0010]FIG. 3 is a plot of the atomic oxygen fraction as a function ofvacuum chamber pressure at two different discharge voltages in theatomic oxygen polishing plasma source of the present invention;

[0011]FIG. 4a is a plot of the fraction of atomic oxygen produced by thepolishing source as a function of discharge voltage in an embodiment ofthe present invention without magnetic filtration;

[0012]FIG. 4b is a plot of the fraction of atomic oxygen produced by thepolishing source as a function of discharge voltage in an embodiment ofthe present invention having magnetic filtration;

[0013]FIG. 4c is an overlay of FIG. 4a and FIG. 4b illustrating thecontrast between atomic oxygen concentrations in embodiments of thepresent invention with and without magnetic filtration; and

[0014]FIG. 5 is a schematic representation of the chemical etching of adiamond surface by atomic oxygen in accordance with the presentinvention.

DETAILED DESCRIPTION

[0015] The present invention is useful for providing an apparatus forlarge scale diamond polishing. The following description is presented toenable one of ordinary skill in the art to make and use the invention,which may be incorporated in the context of a variety of applications.Various modifications to the preferred embodiment, as well as a varietyof uses in different applications will be readily apparent to thoseskilled in the art. Notably, the general principles defined herein maybe applied to other embodiments; thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and novel featuresdisclosed herein.

[0016] A schematic of the preferred embodiment of the atomic oxygenpolishing plasma source of the present invention is outlined in FIG. 1.The atomic oxygen polishing plasma source 100 is a hollow body, whichmay be cylindrical, having one open end, henceforth referred to as theplasma source exit 102, a wall 104, a closed end 106, and an interiorregion 107 a which forms a reaction chamber 107. The wall 104 and theclosed end 106 of the plasma source 100 include a cylindrical array ofconfinement magnets 108 that are held by their own magnetic fields tothe inside surface 110 a of a metal cylinder 110 made of a magneticmaterial such as low carbon steel. A cooling jacket 111 made of anon-magnetic material such as stainless steel completely encases thecylindrical array of confinement magnets 108. The apparatus mayoptionally also include a shield located between the cooling jacket 111and the reaction chamber 107 made of an oxidant-resistant material suchas molybdenum. An electron source 112, powered by an AC voltage source114, is inserted through the closed end 106 into the reaction chamber107 of the plasma source 100. For the purposes of clarity, the electronsource 112 will be referred to as a filament. However, it will beobvious to one skilled in the art that other electron sources would besuitable in other embodiments. In addition, the filament electron source112 and plasma source wall 104 are connected to a DC discharge powersupply 118 located outside the plasma source. The plasma source is alsoequipped with a leak valve or gas port 120 at the closed end 106. Aplanar array of filtration magnets 122 is located within the plasmasource reaction chamber 107, aligned parallel to the plasma source exit102. A planar transparent electrode grid 126 covers the plasma sourceexit 102. The diamond sample, or samples, of interest 130 are placedbeyond the plasma source exit 102.

[0017] In the preferred embodiment, the atomic oxygen polishing plasmasource is positioned inside an evacuated vacuum chamber 200 (shown inFIG. 2). Oxygen gas is introduced into the plasma source 100 through thegas port 120. The final pressure is selected to maximize the atomicoxygen fraction. The pressure in the reaction chamber 107 is theequilibrium pressure created from the flow between the oxygen leakvalve, any gaseous reaction products and the vacuum chamber pump. In oneembodiment of the present invention a final vacuum chamber pressure ofabout 10⁻⁵ to 10⁻³ Torr provided good results. The electron sourcefilament 112, which in the preferred embodiment is made of tungsten,tantalum, or iridium, is heated to thermionic temperatures, at whichpoint electrons ranging in energy from approximately 10 to 100 eV areemitted from the filament 112. The emitted electrons are the primaryelectrons. When a DC bias voltage from the DC power supply 118 isapplied between the filament 112 and the plasma source wall 104, theprimary electrons emitted by the filament are accelerated to the plasmasource wall 104. On their way through the plasma source reaction chamber107, the primary electrons collide with the introduced oxygen molecules,producing primarily molecular oxygen ion plasma and secondary electronsranging in energy from approximately 0 to 3 eV. During the plasmacreation process, the cooling jacket 111 cools the plasma source 100 inorder to prevent heating of the confinement magnets 108 and filtrationmagnets 122 and other potentially destabilizing effects.

[0018] The resultant plasma is comprised of molecular and atomic oxygenspecies, both neutral and ionic, and free electrons. An illustration ofthe plasma composition is presented in FIG. 2. Magnetic filtration isused to increase the fraction of atomic ions present in the plasma asfollows: The magnetic filter 122 creates a transverse magnetic field inthe reaction chamber 107 that prevents the high-energy electronsinvolved in plasma formation from exiting the plasma source 100.However, low energy electrons and positive ions can penetrate the filter122 through a collision diffusion mechanism. In the region 202downstream of the filter 122, the low energy electrons aid in thedissociation of the molecular oxygen ions into atomic ions before theplasma leaves the plasma source 100 as a plume 204 on its way to thediamond surface(s) 130. Dissociation of molecular oxygen ions to atomicions in the downstream region 202 is dependent on the gas pressure inthe vacuum chamber 200.

[0019] In the preferred embodiment, the plume of plasma 204 generated bythe atomic oxygen polishing plasma source 100 is of largely uniformdensity over approximately 12 cm in diameter at the plasma source exit102, and broadens with increasing distance from the plasma source exit102. Thus, with appropriate positioning from the plasma source exit 102,the diamond surface(s) 130 may be completely covered by the atomicoxygen plasma plume 204. Furthermore, because the atomic oxygen ionssimply diffuse from the plasma source 100, the energies of these ionsare largely dependent on collision effects, which are controlledprimarily by gas diffusion and pressure gradients. As a result, theatomic ions in the plasma plume 204 have energies less than about 100eV, rendering them well-suited to chemically etch a diamond sample 130.

[0020] The effect of gas pressure on the fraction of atomic oxygen isshown in FIG. 3. The effect of magnetic filtration on the fraction ofatomic oxygen present in the plasma in the preferred embodiment isillustrated graphically in FIG. 4. As shown in FIG. 4a, the fraction ofatomic oxygen in this embodiment is on the order of 0.28. The datapoints in FIG. 4a were taken at a pressure of 2×10⁻⁴ TORR and at adischarge current of 10 Amps. However, as shown in FIG. 4b, the additionof the magnetic filter 122 increases the fraction of atomic oxygen to atleast 0.7 and in some cases to over 0.94. In FIG. 4b the diamond pointswere taken at a pressure of 4×10⁻⁴ TORR and a discharge current of 40Amps, while the square points were taken at a pressure of 4×10⁻⁴ TORRand a discharge current of 10 Amps. An overlay of the atomic oxygenfraction data with and without the magnetic filter 122 is provided inFIG. 4c. In FIG. 4c, the diamond points represent the non-filter pointsshown in FIG. 4a, while the square points were taken at a pressure of4×10⁻⁴ TORR and a discharge current of 40 Amps, while the trianglepoints were taken at a pressure of 4×10⁻⁴ TORR and a discharge currentof 10 Amps.

[0021] The present invention takes advantage of the ability of atomicoxygen to oxidatively chemically etch diamond surfaces. A schematic ofthe etching mechanism at a diamond surface 500 is provided in FIG. 5. Bygenerating high-density atomic oxygen plasma 502 with magneticfiltration, the atomic oxygen polishing plasma source 100 (shown inFIG. 1) provides for a rapid reaction rate between the atomic oxygenplasma 502 and the diamond surface 500. The density, and accordingly thereaction rate, in the preferred embodiment of the present invention canbe easily controlled by adjusting the gas pressure, the applieddischarge power to the plasma source 100, and the power to the electronsource 112.

[0022] Potential anisotropic effects in diamond polishing are furtherlimited in the present invention by the selective generation of lowenergy atomic oxygen plasma 502. Restricting the plasma flow to lowenergy species limits the physical bombardment of the diamond surface500 by atomic or molecular oxygen ions, a process that results indirectional, and accordingly non-uniform, etching of the diamondsurface. Under low energy conditions, the primary etching effect on thediamond surface 500 is the non-directional chemical reaction of atomicoxygen plasma 502 with the diamond surface 500. In addition, because theatomic oxygen polishing plasma source generates low energy plasma 502,the safety hazards associated with high energy beams such as utilized inthe ion-implantation technique described above are minimized.

[0023] Finally, as previously stated, because the diamond is treated inthe present invention by a broad plume of plasma 204 (shown in FIG. 2),it is possible to treat a single large sample in its entirety, orseveral samples 130 simultaneously. The low-energy, high-density atomicoxygen plasma plume 204 diffusing from the source, while alreadynon-directional in its effect, is applied to the entire sample uniformlyto create a smooth, optical quality surface. Thus, the line-of-sighteffects characteristic of beam polishing are effectively eliminated.

1. An apparatus for polishing diamond surfaces by generating atomicoxygen ions in plasma form comprising: a body having a chamber formedtherein, the body having an open end and a power-source end, with theopen end of the body forming a plasma source exit having an exit plane;an array of confinement magnets encircling the body, whereby the bodyand the array of confinement magnets form a plasma generation reactionchamber; an electron source filament connected to an AC power sourcelocated outside the body, said electron source filament being insertedinto the plasma generation reaction chamber; a gas port inserted throughthe power-source end of the body and into the plasma generation reactionchamber; a DC power source located outside the body, and connectedbetween the electron source filament and the body; and an array offiltration magnets positioned near the plasma source exit, and parallelto the plasma source exit plane, said array of filtration magnetsseparating the reaction chamber into an upstream region containing theconfinement magnets and a downstream region.
 2. An apparatus forpolishing diamond surfaces by generating atomic oxygen ions in plasmaform as set forth in claim 1, wherein the electron source filament isformed of a material selected from the group consisting of tungsten,tantalum, and iridium.
 3. An apparatus for polishing diamond surfaces bygenerating atomic oxygen ions in plasma form as set forth in claim 1,wherein the body is formed of low carbon steel.
 4. An apparatus forpolishing diamond surfaces as set forth in claim 1, wherein the plasmais comprised of least 60% atomic oxygen ions.
 5. An apparatus forpolishing diamond surfaces as set forth in claim 1, wherein a dischargevoltage applied between the DC power source and the electron sourcefilament is between 50 and 150 volts.
 6. An apparatus for polishingdiamond surfaces as set forth in claim 1, wherein a pressure of oxygengas introduced into the plasma generation reaction chamber is between6.0×10⁻⁵ and 1.2×10⁻⁴ Torr.
 7. An apparatus for polishing diamondsurfaces by generating atomic oxygen ions in plasma form comprising amagnetic cylinder having a cylindrical chamber formed therein and anopen end and a power-source end, with the open end of the cylinderforming a plasma source exit having an exit plane; a non-magneticcooling jacket formed in a substantially annular and cylindrical shapepositioned within the cylindrical chamber of the magnetic cylinder; asubstantially annular and cylindrical array of confinement magnetsencased within said non-magnetic cooling jacket, whereby the magneticcylinder and the array of confinement magnets form a plasma generationreaction chamber; an electron source filament connected to an AC powersource located outside the magnetic cylinder, said electron sourcefilament being inserted through power-source end of the magneticcylinder and into the plasma generation reaction chamber; a gas portinserted through the power-source end of the magnetic cylinder and intothe plasma generation reaction chamber; a DC power source locatedoutside the magnetic cylinder, and connected between the electron sourcefilament and the magnetic cylinder; and an array of filtration magnetspositioned near the plasma source exit, and parallel to the plasmasource exit plane, said array of filtration magnets separating thereaction chamber into an upstream region containing the confinementmagnets and a downstream region.
 8. An apparatus for polishing diamondsurfaces by generating atomic oxygen ions in plasma form as set forth inclaim 7, wherein the electron source filament is formed of a materialselected from the group consisting of tungsten, tantalum, and iridium.9. An apparatus for polishing diamond surfaces by generating atomicoxygen ions in plasma form as set forth in claim 7, wherein the magneticcylinder is formed of low carbon steel.
 10. An apparatus for polishingdiamond surfaces by generating atomic oxygen ions in plasma form as setforth in claim 7, wherein the non-magnetic cooling jacket is formed ofstainless steel.
 11. An apparatus for polishing diamond surfaces bygenerating atomic oxygen ions in plasma form for polishing diamondsurfaces as set forth in claim 7, further comprising a cylindricalmolybdenum shield located between the non-magnetic cooling jacket andthe plasma generation reaction chamber.
 12. An apparatus for polishingdiamond surfaces as set forth in claim 7, wherein the plasma iscomprised of least 60% atomic oxygen ions.
 13. An apparatus forpolishing diamond surfaces as set forth in claim 7, wherein a dischargevoltage applied between the DC power source and the electron sourcefilament is between 50 and 150 volts.
 14. An apparatus for polishingdiamond surfaces as set forth in claim 7, wherein a pressure of oxygengas introduced into the plasma generation reaction chamber is between6.0×10⁻⁵ and 1.2×10⁻⁴ Torr.